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American Mineralogist; December 1997; v. 82; no. 11-12; p. 1187-1197
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Artificial weathering of the ordinary chondrite Allegan; implications for the presence of Cl (super -) as a structural component in akaganeite

P. A. Bland, S. P. Kelley, F. J. Berry, J. M. Cadogan, and C. T. Pillinger

Open University, Planetary Science Research Institute, Milton Keynes, United Kingdom

A sample of an ordinary chondrite fall, Allegan, was shown by 57 Fe Mossbauer spectroscopy to be unweathered. Using aerated deionized water at 25 degrees and 0 degrees C, and in one case with dissolved salts added, an attempt was made to simulate meteorite weathering processes in the hot and cold desert environments where these samples accumulate. The progress of artificial oxidation was monitored by 57 Fe Mossbauer spectroscopy, and the final products were analyzed by neutron activation. The results confirm weathering mechanisms proposed for naturally weathered meteorites and suggest that temperature is the major factor controlling the stability of the observed oxide mineral assemblage. Akaganeite was observed as a major oxidation product in the experimentally weathered sample. This finding is interesting because it is widely accepted that beta -FeOOH requires Cl (super -) for stability: The sample weathered in deionized water showed only trace amounts of Cl (super -) in both the fresh and weathered meteorite. We suggest that both Cl (super -) and OH (super -) may be effective in filling tunnel sites and that a complete solid solution is possible between these end-members. In addition, a calculated formation age of 4.59+ or -0.05 Ga and a cosmic-ray exposure age of 5.7-6.2 Ma, was derived from neutron activation analyses of the weathered sample. These ages are consistent with independent measurements made on samples of fresh Allegan, although small sample weights mean that the results should be treated with caution. However, the similar ages obtained in the weathered sample indicates that a wider spectrum of cosmochemical analyses is possible with weathered meteorites than previously thought and that congruent dissolution occurs during meteorite weathering.

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